This invention relates generally to digital open-loop sidelobe cancelers and more particularly to a series-iterative configuration of digital open-loop cancelers which allows more than one iteration for effectively canceling interference from a number of sources.
Generally a signal-processing system is designed to reduce the presence of undesirable signals received by its receiver. An example of apparatus for reducing undesirable signals is a canceler. A digital open-loop canceler as described in the U.S. Pat. No. 4,086,592 to Lewis, et al., is advantageous over an analog closed-loop canceler because the digital open-loop canceler is independent of loop gain and auxiliary signal power. However, a system comprising a number N of cancelers is required to cancel a significant amount of interference from a number N of interference sources. Such a system of N cancelers is an iteration. Typically, more than one iteration is required to cancel most or all interference from a number of sources.
The use of existing digital open-loop cancelers in a series-iterative configuration having more than one iteration, as for example the configuration described in U.S. Pat. No. 3,938,153 to Lewis et al., is inappropriate for canceling interference from a number of sources because existing cancelers use a sliding-window sampling technique for measuring the correlation between the main signal and an auxiliary signal. This measurement, or weight, causes subsequent cancelers in subsequent iterations to introduce uncorrelated interference, known as control-loop noise, which is due to fluctuations of the weight, into the main input signal. The control-loop noise cannot be removed by successive cancellations or iterations. Such a disadvantage limits the performance of a canceling system so that optimum cancellation cannot be achieved.